There is proposed, for the purpose of effectively irradiating the object of irradiation with lights from light sources, an illuminator that uses a light guide whose section is a paraboloid (see Patent Document 1 for instance).
There is also known a compound parabolic concentrator (CPC) which is intended to enhance the efficiency of concentration with no regard for image formation performance (Non-Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 2001-330734    Non-Patent Document 1: Title of book—No. 6, Pencil of Light 5: Non-Image Forming Light Concentrating Optical System; Author—Tadao Tsuruta
Lights emitted from a light guide expand. As a result, where the distance between the document and the emitting face is great, the illumination of the face of the document may decline, which is not desirable.
Incidentally, as stated in Non-Patent Document 1, a compound parabolic concentrator (CPC) is an optical system devised for efficient concentration of lights, and has a characteristic that all the lights incident on the light receiving face at an angle of incidence not greater than θ concentrate on the light concentration face.
Then, an object of the present invention is to provide a light guide capable of minimizing the expansion of light by conversely utilizing the characteristic of the compound parabolic concentrator (CPC) to convert scattered lights extending over a full angle from a limited area into radiant lights confined to a prescribed emission angle and using them as an optical system efficiently illuminating the face of the document, and a line illuminator using that light guide.